/**
 *    Copyright 2009-2018 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.scripting.xmltags;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import ognl.Ognl;
import ognl.OgnlException;

import org.apache.ibatis.builder.BuilderException;

/**
 * Caches OGNL parsed expressions.
 *
 * @author Eduardo Macarron
 *
 * @see <a href='http://code.google.com/p/mybatis/issues/detail?id=342'>Issue
 *      342</a>
 */
public final class OgnlCache {

	private static final OgnlMemberAccess MEMBER_ACCESS = new OgnlMemberAccess();
	private static final OgnlClassResolver CLASS_RESOLVER = new OgnlClassResolver();
	private static final Map<String, Object> expressionCache = new ConcurrentHashMap<>();

	private OgnlCache() {
		// Prevent Instantiation of Static Class
	}

	public static Object getValue(String expression, Object root) {
		try {
			// 创建一个OgnlContext对象
			Map context = Ognl.createDefaultContext(root, MEMBER_ACCESS, CLASS_RESOLVER, null);
			// 调用Ognl的api获取值
			// 第一个参数是ognl表达式
			// 第二个参数是OnglContext上下文对象
			// 第三个是根对象
			return Ognl.getValue(parseExpression(expression), context, root);
		} catch (OgnlException e) {
			throw new BuilderException("Error evaluating expression '" + expression + "'. Cause: " + e, e);
		}
	}

	private static Object parseExpression(String expression) throws OgnlException {
		Object node = expressionCache.get(expression);
		if (node == null) {
			node = Ognl.parseExpression(expression);
			expressionCache.put(expression, node);
		}
		return node;
	}

}
